1. Field
Example embodiments relate to an organic polymer semiconductor, an ambipolar organic thin film transistor (ambipolar OTFT) using the same, an electronic device comprising the ambipolar organic thin film transistor and methods of fabricating the same. Other example embodiments relate to an organic polymer semiconductor, which may include an aromatic ring derivative having p-type semiconductor properties and a heteroaromatic ring having n-type semiconductor properties in the main chain thereof, and which thus may exhibit both p-type transistor properties and n-type transistor properties when used in the organic active layer of an electronic device, e.g., an organic thin film transistor (OTFT), an ambipolar OTFT using the same, an electronic device comprising the ambipolar organic thin film transistor and methods of fabricating the same.
2. Description of the Related Art
After the development of polyacetylene, which is a conjugated organic polymer having semiconductor properties, organic semiconductors have started receiving attention as an electric and electronic material thanks to the advantages of organic material, for example, the variety of synthesis methods, easier formability into fibers or films, flexibility, conductivity, and decreased preparation costs, and thus have been intensively and extensively studied in the wide field of functional electronic devices and optical devices.
Among devices using such a conductive polymer, research into OTFTs using organic material as an active layer is being conducted all over the world these days. Compared to conventional Si-based TFTs, OTFTs are advantageous because a semiconductor layer may be formed through a printing process under atmospheric pressure in place of plasma-enhanced chemical vapor deposition, and all of the fabrication processes may be carried out through a roll-to-roll process using a plastic substrate, if necessary, thereby decreasing the cost of fabricating the transistor. Accordingly, the OTFT may be variously applicable to devices for driving active displays, smart cards and/or plastic chips for inventory tags.
Generally, an organic thin film transistor, comprising a substrate, a gate electrode, an insulating layer, source/drain electrodes, and a channel layer, may be classified as a bottom contact type, in which the channel layer is formed on the source/drain electrodes, and a top contact type, in which a metal electrode is formed on the channel layer through mask deposition. In recent years, in the display field, flat panel displays (FPDs), typically represented by liquid crystal displays (LCDs), have been spotlighted. Therefore, in order to fabricate displays, which have larger areas and are inexpensive and flexible, the channel layer of the OTFT may be required to consist of an organic semiconductor material able to be subjected to a low-temperature solution process, instead of an inorganic semiconductor material, e.g., silicon (Si), which is expensive and requires a high-temperature vacuum process.
Moreover, much research effort has been directed toward organic semiconductor material for the channel layer of the OTFT, and also transistor properties thereof have been reported. Examples of low-molecular-weight or oligomer organic semiconductor material may include merocyanine, phthalocyanine, perylene, pentacene, C60 and/or thiopheneoligomer. The use of pentacene monocrystals has been reported to result in increased charge mobility of about 3.2˜5.0 cm2/Vs or more. However, when using low-molecular-weight organic semiconductor, a vacuum deposition process may be mainly applied to form the channel layer.
Although OTFTs fabricated using a thiophene polymer as polymer material have properties undesirable when compared with OTFTs using low-molecular-weight material, they are advantageous in terms of processibility because a larger area may be realized at a decreased price through a solution process, e.g., printing. In this regard, research has already reported the experimental fabrication of a polymer-based OTFT using a polythiophene material, called F8T2, leading to charge mobility of about 0.01˜0.02 cm2/Vs. As mentioned above, the polymer-based organic semiconductor material may have TFT properties, e.g., charge mobility, which may be undesirable when compared with low-molecular-weight material including pentacene. However, the polymer-based organic semiconductor material may eliminate the need for increased operating frequency and enables the inexpensive fabrication of TFTs.
In order to commercialize the OTFTs, an increased on/off ratio, as an important parameter, in addition to charge mobility, should be achieved. Leakage current in an off-state may be minimized or reduced. Thus, various attempts to improve such properties are being made these days.
The related art discloses an OTFT comprising an active layer composed of n-type inorganic semiconductor material and p-type organic semiconductor material to thus slightly improve the properties thereof, which is still difficult to use and realize mass production because the fabrication process is similar to that of a conventional Si-based TFT. In addition, the related art discloses an OTFT having charge mobility of about 0.01˜0.04 cm2/Vs using regioregular polythiophene P3HT. When using P3HT as representative regioregular polythiophene, the charge mobility may be merely about 0.01 cm2/Vs but cut-off leakage current may be increased (about 10−9 A or more), and thus the on/off ratio may be undesirably decreased, to an extent of about 400 or less, consequently making it impossible to serve in an electronic device. Thorough research into organic semiconductor polymers for polymer-based OTFTs satisfying both increased charge mobility and decreased cut-off leakage current has been conducted, but there is only a small number of reported examples.
The organic semiconductor material for OTFTs, which is classified into low-molecular-weight material and polymeric material, may be further divided into p-type material (hole transport), n-type material (electron transport), and charge-transfer (CT) material simultaneously exhibiting the above two properties, depending on the type of carrier. Separately from the type of organic semiconductor material, the transistor is classified into a p-type transistor, an n-type transistor, and an ambipolar or bipolar transistor.
Because almost all of the initially developed semiconductor materials are p-channel type, exhibiting hole transport upon the application of negative voltage, the recent development of polymer semiconductor material has been primarily conducted toward n-type material, or semiconductor material for ambipolar TFTs. With the goal of manufacturing CMOS circuits required for the design of various electronic devices, there is a need for the development of n-type material or semiconductor material for ambipolar transistors, as organic semiconductor material along with p-type semiconductor material.